Add is_active field to EventTheme model and test fixture.

The `is_active` field was introduced to the `EventTheme` model to indicate whether a theme is active. The corresponding test fixture in `conftest.py` was updated to include this new field, ensuring consistency in tests. This change enhances flexibility for managing event themes.

backend/app/models/event_theme.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, String, JSON
+from sqlalchemy import Column, String, JSON, Boolean
 from sqlalchemy.orm import relationship
 from .base import Base, TimestampMixin, UUIDMixin
 
@@ -11,6 +11,7 @@ class EventTheme(Base, UUIDMixin, TimestampMixin):
     preview_image_url = Column(String)
     color_palette = Column(JSON, nullable=False)
     fonts = Column(JSON, nullable=False)
+    is_active = Column(Boolean, default=True, nullable=False)
 
     # Relationship with Event
     events = relationship("Event", back_populates="theme")